<html>
<head>
<meta charset='utf-8'>
<style>
#topbar { background-color:gray; color:blue; padding:10px; margin:10px; }
.classA { background-color:black; color:white; padding:10px; margin:10px; }
.classB { background-color:blue; color:yellow; padding:10px; margin:10px; }
</style>
</head>
<body>

<div id="topbar">頂區塊</div>

<div class="classA">A 區塊</div>

<div class="classB">B 區塊</div>

</body>
</html>